backport hashlib.pbkdf2_hmac per PEP 466 (closes #21304)

Backport by Alex Gaynor.
diff --git a/Lib/test/test_hashlib.py b/Lib/test/test_hashlib.py
index dac945c..3fc172f 100644
--- a/Lib/test/test_hashlib.py
+++ b/Lib/test/test_hashlib.py
@@ -16,6 +16,8 @@
     threading = None
 import unittest
 import warnings
+from binascii import unhexlify
+
 from test import test_support
 from test.test_support import _4G, precisionbigmemtest
 
@@ -382,8 +384,72 @@
 
         self.assertEqual(expected_hash, hasher.hexdigest())
 
+
+class KDFTests(unittest.TestCase):
+    pbkdf2_test_vectors = [
+        (b'password', b'salt', 1, None),
+        (b'password', b'salt', 2, None),
+        (b'password', b'salt', 4096, None),
+        # too slow, it takes over a minute on a fast CPU.
+        #(b'password', b'salt', 16777216, None),
+        (b'passwordPASSWORDpassword', b'saltSALTsaltSALTsaltSALTsaltSALTsalt',
+         4096, -1),
+        (b'pass\0word', b'sa\0lt', 4096, 16),
+    ]
+
+    pbkdf2_results = {
+        "sha1": [
+            # offical test vectors from RFC 6070
+            (unhexlify('0c60c80f961f0e71f3a9b524af6012062fe037a6'), None),
+            (unhexlify('ea6c014dc72d6f8ccd1ed92ace1d41f0d8de8957'), None),
+            (unhexlify('4b007901b765489abead49d926f721d065a429c1'), None),
+            #(unhexlify('eefe3d61cd4da4e4e9945b3d6ba2158c2634e984'), None),
+            (unhexlify('3d2eec4fe41c849b80c8d83662c0e44a8b291a964c'
+                           'f2f07038'), 25),
+            (unhexlify('56fa6aa75548099dcc37d7f03425e0c3'), None),],
+        "sha256": [
+            (unhexlify('120fb6cffcf8b32c43e7225256c4f837'
+                           'a86548c92ccc35480805987cb70be17b'), None),
+            (unhexlify('ae4d0c95af6b46d32d0adff928f06dd0'
+                           '2a303f8ef3c251dfd6e2d85a95474c43'), None),
+            (unhexlify('c5e478d59288c841aa530db6845c4c8d'
+                           '962893a001ce4e11a4963873aa98134a'), None),
+            #(unhexlify('cf81c66fe8cfc04d1f31ecb65dab4089'
+            #               'f7f179e89b3b0bcb17ad10e3ac6eba46'), None),
+            (unhexlify('348c89dbcbd32b2f32d814b8116e84cf2b17'
+                           '347ebc1800181c4e2a1fb8dd53e1c635518c7dac47e9'), 40),
+            (unhexlify('89b69d0516f829893c696226650a8687'), None),],
+        "sha512": [
+            (unhexlify('867f70cf1ade02cff3752599a3a53dc4af34c7a669815ae5'
+                           'd513554e1c8cf252c02d470a285a0501bad999bfe943c08f'
+                           '050235d7d68b1da55e63f73b60a57fce'), None),
+            (unhexlify('e1d9c16aa681708a45f5c7c4e215ceb66e011a2e9f004071'
+                           '3f18aefdb866d53cf76cab2868a39b9f7840edce4fef5a82'
+                           'be67335c77a6068e04112754f27ccf4e'), None),
+            (unhexlify('d197b1b33db0143e018b12f3d1d1479e6cdebdcc97c5c0f8'
+                           '7f6902e072f457b5143f30602641b3d55cd335988cb36b84'
+                           '376060ecd532e039b742a239434af2d5'), None),
+            (unhexlify('8c0511f4c6e597c6ac6315d8f0362e225f3c501495ba23b8'
+                           '68c005174dc4ee71115b59f9e60cd9532fa33e0f75aefe30'
+                           '225c583a186cd82bd4daea9724a3d3b8'), 64),
+            (unhexlify('9d9e9c4cd21fe4be24d5b8244c759665'), None),],
+    }
+
+    def test_pbkdf2_hmac(self):
+        for digest_name, results in self.pbkdf2_results.items():
+            for i, vector in enumerate(self.pbkdf2_test_vectors):
+                password, salt, rounds, dklen = vector
+                expected, overwrite_dklen = results[i]
+                if overwrite_dklen:
+                    dklen = overwrite_dklen
+                out = hashlib.pbkdf2_hmac(
+                    digest_name, password, salt, rounds, dklen)
+                self.assertEqual(out, expected,
+                                 (digest_name, password, salt, rounds, dklen))
+
+
 def test_main():
-    test_support.run_unittest(HashLibTestCase)
+    test_support.run_unittest(HashLibTestCase, KDFTests)
 
 if __name__ == "__main__":
     test_main()